Power semiconductor elements such as a switching element and a diode, are used in circuits for a switching regulator, an inverter, and the like. Such power semiconductor elements are required to achieve the increase in the breakdown voltage and the decrease in the ON-resistance. Between the element breakdown voltage and the ON-resistance, there is a trade-off relationship determined by the element material. The use of a wide bandgap semiconductor as the switching element material can improve this trade-off.
Among these elements, the examples of an element easy to achieve a low ON-resistance include a hetero junction field effect transistor (HFET) having a gallium aluminum nitride (AlGaN)/gallium nitride (GaN) hetero structure. In the HFET, a high mobility in a hetero interface channel and a high concentration of electrons generated by piezo polarization achieve the decrease in the ON-resistance.
However, in the HFET, in order to generate electrons (two-dimensional electron gas) by piezo polarization, the gate threshold voltage typically becomes negative. Therefore, the HFET becomes normally on. In the power electronics applications, such as the switching regulator, for safety reasons, such as the prevention of rush current at turn-on, the gate threshold voltage is preferably positive. That is, in the power electronics applications, a normally-off operation is indispensable.
In order to make the HFET normally-OFF, there are a method to introduce a p-type GaN layer on the lower side of a gate electrode, a method to introduce a recess gate structure, and a method to introduce an MOS-type channel structure. However, the control of the impurity concentration of the p-type GaN layer and the control of the recess depth require complicated process technologies. Therefore, in the conventional HFET, the gate threshold voltage (Vth) easily varies. Moreover, in the MOS-type channel structure, although the variation of the gate threshold voltage is suppressed, an inversion channel is formed in an MIS gate interface, resulting in the decrease in the mobility and the increase in the ON-resistance.